1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image transferred on a sheet to the sheet and an image forming apparatus employing such a fixing device.
2. Description of the Background Art
There has been conventionally known a fixing device used in an image forming apparatus such as a printer or copier for fixing a toner image transferred on a sheet to the sheet. In such a fixing device, elastic layers (rubber layers) have been provided on a pressure(pressing) roller and a fixing (heating) roller, and means for separating a sheet from the outer circumferential surface of the fixing roller by the shape of a nip between the pressure roller and the fixing roller has been used.
However, if the fixing roller is provided with the elastic layer, problems such as a reduction in temperature following capability when sheets are successively fed and an increase in warm-up time arise due to a lower heat conductivity of the elastic layer. Thus, it has been difficult to sufficiently cope with a recent energy-saving tendency. Further, a technology of fixing a toner image to a sheet using a belt has been proposed and commercialized, but has problems of a complicated construction and an increased cost.
On the other hand, there has been known a fixing device using, as a fixing roller, a so-called hard roller constructed by covering the outer circumferential surface of an aluminum or iron core with a fluorocarbon-resin coating or tube. The warm-up time can be sufficiently shortened due to a high heat conductivity of the fixing roller in this fixing device and it is possible to inexpensively construct the fixing device.
However, since a large adhesive force is created between the fixing roller and the sheet where a large amount of molten toner is present if the above fixing roller (hard roller) is used, it is necessary to use means for forcibly separating the sheet from the outer circumferential surface of the fixing roller by bringing a separating claw into contact with the fixing roller. Therefore, there has been a problem of the abrasion of the fixing roller in the case of repeated use.
Accordingly, in order to suppress the abrasion of the fixing roller by the separating claw, a technology of reducing a contact pressure of the separating claw with the fixing roller by widening the width of the leading end of the separating claw has been proposed, for example, in Japanese Unexamined Patent Publication No. 2004-157481. This publication discloses a fixing device constructed such that a separating claw rotatable about one axial direction (direction of the central axis of a fixing roller) is held in contact with the outer circumferential surface of the fixing roller using a spring.
However, according to such a method for bringing the separating claw into contact with the fixing roller, the separating claw and the fixing roller are substantially in point contact with each other if one side of the leading end of the separating claw is held in contact with the outer circumferential surface of the fixing roller due to dimensional tolerance and the like of components. Thus, a contact pressure of the separating claw with the fixing roller concentrates on this contact point and an effect of widening the width of the leading end of the separating claw cannot be sufficiently obtained, thereby causing a problem of making it difficult to sufficiently suppress the abrasion of the fixing roller. As a result, lines are formed on an image due to the abrasion marking of the fixing roller. Particularly, in a color image forming apparatus, there is a problem of considerable image deterioration.